Searle was born in Cambridge, England, where his father was a porter at Cambridge Railway Station.[2] He started drawing at the age of five and left school at the age of 15. He trained at Cambridge College of Arts and Technology (now Anglia Ruskin University) for two years.

In April 1939, realizing that war was inevitable, he abandoned his art studies to enlist in the Royal Engineers. In January 1942, he was stationed in Singapore. After a month of fighting in Malaya, Singapore fell to the Japanese, and he was taken prisoner along with his cousin Tom Fordham Searle. He spent the rest of the war a prisoner, first in Changi Prison and then in the Kwai jungle, working on the Siam-Burma Death Railway. Searle contracted both beri-beri and malaria during his incarceration, which included numerous beatings, and his weight dropped to less than 40 kilograms. He was liberated in late 1945 with the final defeat of the Japanese. Immediately after the war, he served as a courtroom artist at the Nuremberg trials.

He married the journalist Kaye Webb in 1947; they had twins, Kate and Johnny. In 1961, he moved to Paris, leaving his family and later marrying Monica Koenig, a painter, theatre and jewellery designer.[3] After 1975, Searle and his wife lived and worked in the mountains of Haute Provence.

His wife Monica died in July 2011 and Searle died on 30 December 2011, aged 91.

Although Searle published the first St Trinian's cartoon in the magazine Lilliput in 1941, his professional career really begins with his documentation of the brutal camp conditions of his period as a prisoner-of-war of the Japanese in World War II in a series of drawings that he hid under the mattresses of prisoners dying of cholera. Searle recalled, "I desperately wanted to put down what was happening, because I thought if by any chance there was a record, even if I died, someone might find it and know what went on." But Searle survived, along with approximately 300 of his drawings. Liberated late in 1945, Searle returned to England where he published several of the drawings in fellow prisoner Russell Braddon's The Naked Island. Another of Searle's fellow prisoners later recounted, "If you can imagine something that weighs six stone or so, is on the point of death and has no qualities of the human condition that aren’t revolting, calmly lying there with a pencil and a scrap of paper, drawing, you have some idea of the difference of temperament that this man had from the ordinary human being."[4]

Most of these drawings appear in his 1986 book, Ronald Searle: To the Kwai and Back, War Drawings 1939-1945.[5] In the book, Searle also wrote of his experiences as a prisoner, including the day he woke up to find a dead friend on either side of him, and a live snake underneath his head:

"You can’t have that sort of experience without it directing the rest of your life. I think that’s why I never really left my prison cell, because it gave me my measuring stick for the rest of my life... Basically all the people we loved and knew and grew up with simply became fertiliser for the nearest bamboo."

After moving to Paris in 1961, he worked more on reportage for Life and Holiday and less on cartoons. He also continued to work in a broad range of media and created books (including his well-known cat books), animated films and sculpture for commemorative medals, both for the French Mint and the British Art Medal Society.[7][8] Searle did a considerable amount of designing for the cinema, and in 1965, he completed the opening, intermission and closing credits for the comedy film Those Magnificent Men in Their Flying Machines as well as the 1969 film Monte Carlo or Bust. In 1975, the full-length cartoon Dick Deadeye, or Duty Done was released. It is based on the character and songs from H.M.S. Pinafore.[9] Animated by a number of artists both British and French, it is considered by some to be his greatest achievement, although Searle himself detested the result.

In 2010, he gave about 2,200 of his works as permanent loans to Wilhelm Busch MuseumHannover (Germany), now renamed Deutsches Museum für Karikatur und Zeichenkunst. The ancient Summer palace of George 1st, this Museum holds Searle's archives.

Down With Skool!: A Guide to School Life for Tiny Pupils and Their Parents, 1953 (with Geoffrey Willans)

How to be Topp: A Guide to Sukcess for Tiny Pupils, Including All There is to Kno About Space, 1954 (with Geoffrey Willans)

Whizz for Atomms: A Guide to Survival in the 20th Century for Fellow Pupils, their Doting Maters, Pompous Paters and Any Others who are Interested, 1956 (with Geoffrey Willans) Published in the U.S. as Molesworth's Guide to the Atommic Age